A sensor that determines information related to a gaseous analyte in a body of fluid. The sensor comprises an emitter, a luminescable medium, a radiation sensor, and a processor. The emitter emits electromagnetic radiation having an oscillating intensity. The luminescable medium communicates with the body of fluid and emits luminescent radiation in response to the received electromagnetic radiation. The radiation sensor receives the luminescent radiation, and generates an output signal based on the intensity of the received luminescent radiation. The processor samples the output signal generated by the radiation sensor at two or more predetermined periodic points over the oscillation of the intensity of the electromagnetic radiation to determine information from the samples related to a phase difference between the oscillation of the intensity of the electromagnetic radiation emitted by the emitter and oscillation of the intensity of the luminescent radiation received by the radiation sensor.